Well, I'm not sure if this is a special configuration they must enable or not, but usually you can just place a custom php.ini file in the root of your website and it will apply to all of your PHP scripts.
The catch is, however, that you must have a complete php.ini file - not just a few lines that you wish to change. As such, you'd need to obtain a copy of their current php.ini file, make the changes you want, and then save it to the root of your website.
If you do a phpinfo() and find out where their php.ini file is stored, you may be able to use [man]file_get_contents/man in a PHP script to grab it and save it to your web root.